The performance of a photonics integrated circuit (PIC) is often polarization-dependent. For example, individual building blocks of a PIC may be specifically designed for a particular polarization. Therefore, for efficient operation of the device, it is known in the art to pre-emptively adjust the polarization of the input signal such as to match the polarization for which the PIC building blocks are designed.
As real life signals are often arbitrarily polarized, it would be advantageous to make the PIC performance of a photonics integrated circuit independent of the polarization status of the input signal, or to at least reduce the impact of polarization on the operation of the photonic device.
One solution for this problem that is known in the art is to adopt a “polarization diversity” approach, e.g. as illustrated in FIG. 1. Assuming a polarization independent interface between an external optical fiber and the PIC, and assuming that the PIC building blocks are for example designed for the TE polarization, this approach for polarization status independent PIC operation comprises splitting the input polarization into two orthogonal polarizations (TE and TM), rotating the TM over 90 degrees such as to transform it into a TE polarization, and then processing the two TE polarizations by means of two identical replicas of the original PIC. The signals can be eventually recombined after one of the two TE polarizations is transformed back into TM. However, this approach has the disadvantage of doubling the PIC real estate, and thus increasing the cost, size and power consumption of the device. Furthermore, processing the two polarizations independently by means of two replicas of the original PIC also has the disadvantage that the two resulting signals will differ due to differences between the two PIC replicas, e.g. due to fabrication tolerances. This affects the quality of the recombined signal at the output of the device.
The PIC interfaces for coupling optical signals in and out of the chip may furthermore be designed for a specific polarization. As a consequence, in order to couple an external signal efficiently into the PIC, the signal polarization must be adjusted such to match the polarization for which the interface is designed. For example, 1D grating couplers may provide an efficient interface between a PIC and an external optical fiber for one specific polarization.
2D grating couplers allow the coupling of an external signal from and to the PIC independently of its polarization, e.g. as disclosed in European Patent Application EP 1353200. Thus, both orthogonal input polarizations may be coupled efficiently into the PIC, while automatically splitting them and rotating one polarization over 90 degrees. Thus the approach illustrated in FIG. 1 can be implemented using 2D grating couplers as IN/OUT chip interfaces, e.g. as illustrated in FIG. 2. Such 2D grating couplers have the advantage of implementing automatically the polarization splitting and polarization rotation, which otherwise would have to be implemented using integrated polarization splitters and rotators. Such integrated splitters and rotators as known in the art may have a poor on chip-performance. Nevertheless, the problem of signal impairment due to the fabrication differences between the two PIC replicas used to process the two polarizations independently remains present.